In many industrial processes, high temperatures are encountered by the apparatus used in those processes. Those temperatures are so high that continued contact of the material being processed with the apparatus causes substantial deterioration of the apparatus. In such cases, a high temperature resistant, insulating inorganic paper is used to protect the apparatus from the high temperatures. While these materials are referred to in the art as "papers", since they resemble wood pulp papers in that they are composed of interlocked staple fibers, these papers are not made of organic fibers but are made of inorganic fibers, particularly certain types of ceramic fibers. In addition, these papers are made on conventional paper making machines and in that sense are also similar to wood pulp papers.
Examples of applications of the present papers are where the papers are used to line a rotary kiln so that the paper is disposed between the steel kiln shell and the fire bricks of the kiln to protect the shell. Another example is where the papers are used to line a metal trough which carries molten metal. As can be appreciated from these examples, the papers must be very high temperature resistant, but must also be capable of being configured during installation of the papers in the apparatus into different shapes which approximate the shape of the apparatus being protected. Unfortunately, however, since the papers are made of inorganic fibers, usually certain types of ceramic fibers, as opposed to organic fibers such as cotton, wood pulp, wool, and plastic fibers, the fibers of the papers have relatively smooth exterior surfaces and are most often of relatively short staple length. As a result thereof, when the inorganic fibers are interlocked together to form a paper, the interlocking of the relatively smooth exterior surfaced fibers is not nearly as great as the interlocking achieved with organic fibers. This results in the paper being of relatively low strength, both during processing and in the finished form ready for installation in an apparatus. Due to this low strength, it is both difficult to process the papers and to configure the finished papers into a shape appropriate for the apparatus being protected. As noted above, the usual inorganic fibers are certain types of ceramic fibers and these ceramic fibers present particular problems in the above regards, since the interlocking of these ceramic fibers is particularly poor due to the rigid nature of those fibers and the very short staple lengths thereof.
In order to provide sufficient strength to these ceramic papers during both processing and configuration to apparatus, a binder is normally placed in the ceramic papers. More usually, this binder is placed in the ceramic fiber mix from which the paper is made so that the binder will improve the strength of the papers during the processing thereof. Otherwise, it is difficult to process the ceramic papers, and paper breaking during processing is a continual problem. In addition, since these ceramic papers are processed on ordinary paper making machines in a continuous manner, such breaking of the paper during processing considerably increases the cost of producing the papers, due to lost production and down time of processing equipment. The binder also increases the strength of the finished paper so it may be cut and configured, without substantial breaking, during application to an apparatus to be protected.
While inorganic binders have been proposed in the art, inorganic binders are not particularly effective, either in improving the strength of the papers during processing or in regard to configuring the papers in application to an apparatus to be protected. Accordingly, primarily, the art uses organic binders in the ceramic papers. These organic binders take a variety of forms, but primarily the binders are polymer compositions, such as compositions formed of phenolics, acrylics, epoxies, polyvinylchloride, polyvinylacetate/alcohol, and the like. These binders function quite satisfactorily to improve the structural integrity, and hence the strength, of the ceramic papers, when the papers are used at lower temperatures. However, when the papers are used in environments with temperatures at about the 300.degree. to 400.degree. F. range, these binders begin to lose their binding effect, with a concomitant loss of structural integrity. With continued use at these temperatures, the binders will lose essentially all of their binding effect and the structural integrity of the paper will then be essentially only that integrity provided by the interlocking of the ceramic fibers. At higher temperatures, these binders will very quickly burn away and the paper will have the structural integrity only of that provided by the interlocked ceramic fibers.
As a result thereof, when the ceramic papers are intended to be used in high temperature environments, i.e. environments where the temperature is about 400.degree. F. or higher, the strength provided by these binders is very quickly dissipated, and it is necessary for other provisions to be made such that the ceramic papers, with the considerably decreased structural integrity, may nevertheless function for the insulating properties required. To achieve this end, various mechanical devices and like arrangements have been suggested in the art. For example, in rotary kilns, by placing the ceramic paper between the steel kiln shell and the fire bricks, in certain manners, the fire bricks can lock the ceramic paper to the kiln shell and hold it in place even after the binder has burned away and the ceramic paper has considerably reduced structural integrity. The binder, therefore, functions to allow the ceramic paper to be processed and subsequently configured to the shape of the kiln until locked in place, during rotation of the kiln, by the fire bricks. At that point, the reduced integrity of the ceramic paper will not be a substantial problem, even after the binder has completely burned away.
While the foregoing approach is quite prevalent in the art and is reasonably successful for certain applications, that approach has serious disadvantages in connection with a number of industrial processes. As is appreciated from the foregoing, during the initial operation of the apparatus, the binder burns away and the gases produced from the binder are first contained in the apparatus and, in time, dissipated therefrom. However, there are many industrial processes where organic combustion products cannot be tolerated. Thus, in those processes, when the binder is being burned away, the product of those processes is unacceptably contaminated with the combustion products of the organic binder. In such processes, ceramic papers with binders are totally unacceptable. However, without binders the ceramic papers, as noted above, are very difficult to produce and more difficult to configure to the apparatus to be protected without breaking or seriously damaging the ceramic paper.
Thus, it would be a significant advantage to the art to provide ceramic papers where the papers have improved structural integrities and strengths during processing and during configuration, but where those papers do not produce organic combustion products when used in high temperature environments. Heretofore, the art has not been able to provide such ceramic papers.